Many implantable medical device systems use magnetically coupled antennas for one or both of telemetry communications and inductive recharging of a power source of the implantable medical device. In both scenarios, the relative positioning of the primary and secondary, i.e., external and internal, antennas is important to the performance of the system. This is especially true for recharging, where the positioning of the primary antenna has a dramatic impact on the efficiency of the recharge procedure and on the associated time that must be spent recharging. Localization of an external device relative to an implanted object can also be very important in, for example, implantable drug pump systems, where it may be necessary to locate a subcutaneous refill port.
Some existing implantable medical device systems incorporate an antenna, or other implanted object, locating feature. This feature has variously been implemented using a signal strength approach, e.g., measuring the strength of a telemetry signal sent by an implantable medical device to the external antenna, or by using a metal detection approach, e.g., measuring the loading of the external antenna caused by the proximity of the implant. However, both of these techniques are limited to providing an output proportional to the distance of separation. This output, while somewhat useful in guiding the user into better placement, does not contain enough information to generate precise guidance.
Systems have also been built that locate an implanted object using multiple external antennas to triangulate the position of implanted object, e.g., an internal antenna. This type of system can provide more detailed guidance to a user for location, but only at the expense of additional external hardware, and additional consumption of power to drive the additional antennas.